 Originally CKD records had a one-to-one correspondence to a physical track of a DASD device; however over time the records have become more and more virtualized such that in modern IBM mainframes there is no longer a direct correspondence between the a CKD record ID and a physical layout of a track.Access to specific classes of I/O devices by an IBM mainframe is under the control of Channel Command Words (CCWs), some of which are generic (e.g.No Operation) but many of which are specific to the type of I/O device (e.g.